Ink set

ABSTRACT

An ink set comprising a black ink and a color ink, wherein the black ink comprises an aqueous dispersion of water-insoluble vinyl polymer particles containing an oil-soluble black dye or carbon black, the vinyl polymer particles have an ionicity opposite to that of the color ink, and the vinyl polymer comprises a water-insoluble vinyl polymer prepared by copolymerizing (a) a salt forming group-containing monomer, (b) a monomer represented by the formula (I):
 
CH 2 ═C(R 1 )COO(R 2 O) p R 3    (I)
 
wherein R 1  is hydrogen atom or a lower alkyl group, R 2  is a divalent hydrocarbon group which may have a hetero-atom, R 3  is a monovalent hydrocarbon group which may have a hetero-atom, and p is a number of 1 to 60, (c) a macromer, and (d) a monomer copolymerizable with these monomers. The ink set can be used as an ink set for inkjet recording, which is excellent in not only fixing abilities and water resistance, but also bleeding resistance and jettability, and can form very clear printouts.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/JP00/09273 which has an Internationalfiling date of Dec. 26, 2000, which designated the United States ofAmerica.

TECHNICAL FIELD

The present invention relates to an ink set. More specifically, thepresent invention relates to an ink set which can be suitably used foran ink set for inkjet recording.

BACKGROUND ART

Inkjet recording is a recording system for forming characters or images,comprising directly jetting ink droplets from very fine nozzles to arecording medium, and depositing the ink droplets on the recordingmedium. This system has advantages that not only the used device showsexcellent operability at a low level of noise, but also the colorationis facilitated and plain paper can be used as a recording medium.Therefore, this system has been widely used in the recent years.

As an ink set comprising a black ink and a color ink usable for colorinkjet printers, there have been known an ink set comprising an acidicblack ink comprising an acidic carbon black, a basic surfactant and awater-soluble organic solvent, and a basic color ink (Japanese PatentLaid-Open No. Hei 9-183224); an ink set comprising a black inkcomprising a self-dispersible carbon black in which at least onehydrophilic group is bonded directly or via the other group of atoms tothe surface of the carbon black, and a color ink comprising a coloranthaving a polarity opposite to the colorant used in the black ink(Japanese Patent Laid-Open No. Hei 10-140064); and the like.

However, the printouts formed by using these inks are wrong especiallyin fixing abilities such as rubbing resistance and high-lighter fastnessin the black ink, and are not sufficiently satisfactory in theseproperties, while printouts formed by using there ink sets are excellentin bleeding resistance.

DISCLOSURE OF INVENTION

An object of the present invention is to provide an ink set for inkjetrecording, which is excellent in not only fixing abilities and waterresistance, but also bleeding resistance and jettability, and can formvery clear printouts.

The present invention relates to an ink set comprising a black ink and acolor ink, wherein the black ink comprises an aqueous dispersion ofwater-insoluble vinyl polymer particles containing an oil-soluble blackdye or carbon black, the vinyl polymer particles have an ionicityopposite to that of the color ink, and the vinyl polymer comprises awater-insoluble vinyl polymer prepared by copolymerizing (a) a saltforming group-containing monomer [hereinafter referred to as component(a)], (b) a monomer represented by the formula (I):CH₂═C(R¹)COO(R²O)_(p)R³  (I)wherein R¹ is hydrogen atom or a lower alkyl group, R² is a divalenthydrocarbon group which may have a hetero-atom, R³ is a monovalenthydrocarbon group which may have a hetero-atom, and p is a number of 1to 60 [hereinafter referred to as component (b)], (c) a macromer[hereinafter referred to as component (c)], and (d) a monomercopolymerizable with these monomers [hereinafter referred to ascomponent (d)].

BEST MODE FOR CARRYING OUT THE INVENTION

As a vinyl polymer used in the aqueous dispersion, a water-insolublevinyl polymer is used. In order to obtain a water-based ink having bothhigh fixing ability and high dispersion stability, there is used awater-insoluble vinyl polymer obtained by copolymerizing the component(a), the component (b), the component (c) and the component (d).

The salt-forming group refers to a group which can form a cationic groupor an anionic group when the group is neutralized with an acid or abase.

Representative examples of the component (a) include the cationicmonomers and the anionic monomers described in page 5, column 7, line 24to column 8, line 29 of Japanese Patent Laid-Open No. Hei 9-286939.

The cationic monomer includes tertiary amine-containing unsaturatedmonomers, ammonium salt-containing unsaturated monomers, and the like.Among them, N,N-dimethylaminoethyl acrylate, N,N-dimethylaminoethylmethacrylate, N-(N′,N′-dimethylaminopropyl) acrylamide,N-(N′,N′-dimethylaminopropyl) methacrylamide and vinylpyrrolidone arepreferable.

The anionic monomer includes unsaturated carboxylic acid monomers,unsaturated sulfonic acid monomers, unsaturated phosphoric acidmonomers, and the like. Among them, unsaturated carboxylic acidmonomers, especially acrylic acid and methacrylic acid are preferable.

The component (b) exhibits an excellent effect of suppressing thegeneration of distortion even when printing is continuously carried outbecause the component (b) increases the jetting stability of thewater-based ink of the present invention.

In the formula (I), R¹ is hydrogen atom or a lower alkyl group. Thelower alkyl group includes linear or branched alkyl groups having 1 to 4carbon atoms.

R² is a divalent hydrocarbon group which may have a hetero-atom. Thehetero-atom includes, for example, nitrogen atom, oxygen atom, a halogenatom and sulfur atom.

Representative examples of R² include an aromatic ring having 6 to 30carbon atoms, which may have a substituent; a heterocyclic ring having 3to 30 carbon atoms, which may have a substituent; and an alkylene grouphaving 1 to 30 carbon atoms, which may have a substituent. These ringsor groups may be used in combination of at least two kinds. Thesubstituent includes an aromatic ring having 6 to 29 carbon atoms; aheterocyclic ring having 3 to 29 carbon atoms; an alkyl group having 1to 29 carbon atoms; a halogen atom; amino group, and the like.

Preferred examples of R² include a phenylene group which may have asubstituent having 1 to 24 carbon atoms; an aliphatic alkylene grouphaving 1 to 30 carbon atoms, preferably 1 to 20 carbon atoms; anaromatic ring-containing alkylene group having 7 to 30 carbon atoms; anda heterocyclic ring-containing alkylene group having 4 to 30 carbonatoms.

Preferred examples of R²O group include alkylene oxide groups having 2to 7 carbon atoms, such as ethylene oxide group, (iso)propylene oxidegroup, tetramethylene oxide group, heptamethylene oxide group,hexamethylene oxide group, and combinations of at least two kinds ofthese alkylene oxide groups; phenylene oxide group; and the like.

R³ is a monovalent hydrocarbon group which may have a hetero-atom. Thehetero-atom includes nitrogen atom, oxygen atom and sulfur atom.

Representative examples of R³ include an aromatic ring having 6 to 30carbon atoms, which may have a substituent; a heterocyclic ring having 3to 30 carbon atoms, which may have a substituent; and an alkyl grouphaving 1 to 30 carbon atoms, which may have a substituent. Thesubstituent includes an aromatic ring having 6 to 29 carbon atoms; aheterocyclic ring having 4 to 29 carbon atoms; a halogen atom; aminogroup, and the like.

Preferred examples of R³ include phenyl group, an aliphatic alkyl grouphaving 1 to 30 carbon atoms, preferably 1 to 20 carbon atoms, anaromatic ring-containing alkyl group having 7 to 30 carbon atoms, and aheterocyclic ring-containing alkyl group having 4 to 30 carbon atoms.

More preferred examples of R³ include alkyl groups having 1 to 6 carbonatoms, such as methyl group, ethyl group, (iso)propyl group, (iso)butylgroup, (iso)pentyl group and (iso)hexyl group; phenyl group, and thelike.

p is a number of 1 to 60, out of which a number of 1 to 30 ispreferable.

Concrete examples of the monomer represented by the formula (I) includemethoxypolyethylene glycol (1-30: p value in the formula (I),hereinafter referred to the same) (meth)acrylates,methoxypolytetramethylene glycol (1-30) (meth)acrylates,ethoxypolyethylene glycol (1-30) (meth)acrylates,(iso)propoxypolyethylene glycol (1-30) (meth)acrylates,butoxypolyethylene glycol (1-30) (meth)acrylates, methoxypolypropyleneglycol (1-30) (meth)acrylates, propoxypolypropylene oxide (1-30)(meth)acrylates, methoxy(ethylene glycol-propylene glycol copolymer)(1-30, out of which ethylene glycol: 1-29) (meth)acrylates, and thelike. These monomers can be used alone or in admixture of at least twokinds. Among them, methoxypolyethylene glycol (1-30) (meth)acrylates arepreferable. Incidentally, the term “(meth)acrylate” as used herein meansacrylate or methacrylate. Also, the term “(iso)propoxy” as used hereinmeans n-propoxy or isopropoxy.

The component (c) includes macromers which are monomers having apolymerizable unsaturated group and a number-average molecular weight of500 to 100000, preferably 1000 to 10000. Among them, a silicone macromerrepresented by the formula (II):X(Y)_(q)Si(R⁴)_(3-r)(Z)_(r)  (II)wherein X is a polymerizable unsaturated group; Y is a divalent group;each of R⁴ is independently hydrogen atom, a lower alkyl group, an arylgroup or an alkoxy group, respectively; Z is a monovalent siloxanepolymer residue having a number-average molecular weight of at least500; q is 0 or 1; and r is an integer of 1 to 3,and/or a styrenic macromer having a polymerizable functional group atone end is preferable.

The number-average molecular weight of the macromer is determined by gelchromatography using polystyrene as a standard substance and chloroformcontaining 1 mmol/L of dodecyldimethylamine as a solvent.

The silicone macromer can be favorably used from the viewpoint ofpreventing scorching on the printer head of an inkjet printer.

In the silicone macromer represented by the formula (II), X includes amonovalent unsaturated hydrocarbon group having 2 to 6 carbon atoms,such as CH₂═CH— group and CH₂═C(CH₃)— group. Y includes divalent groupssuch as —COO— group, a —COOC_(a)H_(2a)— group, wherein a is an integerof 1 to 5, and a phenylene group. Among them, —COOC₃H₆— is preferable.R⁴ includes hydrogen atom; a lower alkyl group having 1 to 5 carbonatoms, such as methyl group and ethyl group; an aryl group having 6 to20 carbon atoms, such as phenyl group; an alkoxy group having 1 to 20carbon atoms, such as methoxy group, and the like. Among them, methylgroup is preferable. Z is preferably a monovalent dimethylsiloxanepolymer residue having a number-average molecular weight of 500 to 5000.q is 0 or 1, and preferably 1. r is an integer of 1 to 3, and preferably1.

Representative examples of the silicone macromer include a siliconemacromer represented by the formula (II-1):CH₂═CR⁵—COOC₃H₆—[Si(R⁶)₂—O]_(b)—Si(R⁶)₃  (II-1)wherein R⁵ is hydrogen atom or methyl group; each of R⁶ is independentlyhydrogen atom or a lower alkyl group having 1 to 5 carbon atoms,respectively; and b is a number of 5 to 65;

-   a silicone macromer represented by the formula (II-2):    CH₂═CR⁵—COO—[Si(R⁶)₂—O]_(b)—Si(R⁶)₃  (II-2)    wherein R⁵, R⁶ and b are the same as defined above;-   a silicone macromer represented by the formula (II-3):    CH₂═CR⁵—Ph—[Si(R⁶)₂—O]_(b)—Si(R⁶)₃  (II-3)    wherein Ph is a phenylene group; and R⁵, R⁶ and b are the same as    defined above;-   a silicone macromer represented by the formula (II-4):    CH₂═CR⁵—COOC₃H₆—Si(OE)₃  (II-4)    wherein R⁵ is the same as defined above; E is a group represented by    the formula: —[Si(R⁵)₂O]_(c)—Si(R⁵)₃, wherein R⁵ is the same as    defined above, and c is a number of 5 to 65;    and the like.

Among them, the silicone macromer represented by the formula (II-1) ispreferable, and a silicone macromer represented by the formula (II-1a):CH₂═C(CH₃)—COOC₃H₆—[Si(CH₃)₂—O]_(d)—CH₃  (II-1a)wherein d is a number of 8 to 40,is especially preferable. Examples of the silicone macromer include asilicone macromer manufactured by CHISSO CORPORATION under the tradename of FM-0711, and the like.

The styrenic macromer can be favorably used, from the viewpoint ofsufficient inclusion of an oil-soluble black dye or carbon black in thevinyl polymer.

The styrenic macromer includes styrene homopolymers having apolymerizable functional group at one end, or copolymers of styrene withthe other monomer. Among them, those having acryloyloxy group ormethacryloyloxy group as a polymerizable functional group at one end arepreferable. It is desired that the content of styrene in theabove-mentioned copolymer is at least 60% by weight, preferably at least70% by weight, from the viewpoint of sufficient inclusion of carbonblack in the vinyl polymer. The other monomer mentioned above includesacrylonitrile, and the like.

It is preferable that the number-average molecular weight of thestyrenic macromer is 1000 to 10000.

The component (d) includes (meth)acrylates such as methyl(meth)acrylate, ethyl (meth)acrylate, (iso)propyl (meth)acrylate, (isoor tertiary)butyl (meth)acrylate, (iso)amyl (meth)acrylate, cyclohexyl(meth)acrylate, benzyl (meth)acrylate, 2-ethylhexyl (meth)acrylate,(iso)octyl (meth)acrylate, (iso)decyl (meth)acrylate, (iso)dodecyl(meth)acrylate, (iso)stearyl (meth)acrylate, 2-hydroxyethyl(meth)acrylate and 3-hydroxypropyl (meth)acrylate; styrenic monomerssuch as styrene, vinyltoluene, 2-methylstyrene and chlorostyrene; andthe like. These can be used alone or in admixture of at least two kinds.Incidentally, “(iso or tertiary)butyl” means n-butyl, isobutyl ortertiary butyl. Also, “(iso)” means n- or iso.

It is preferable that the component (d) comprises a styrenic monomerfrom the viewpoints of improvements in optical density and high-lighterfastness. As the styrenic monomer, styrene and 2-methylstyrene arepreferable. These styrenic monomers can be used alone or in admixturethereof. In this case, it is desired that the content of the styrenicmonomer in the component (d) is 10 to 100% by weight, preferably 40 to100% by weight, from the viewpoints of improvements in optical densityand high-lighter fastness.

It is preferable that the component (d) comprises a hydroxylgroup-containing monomer from the viewpoint of jetting stability in aninkjet printer. As the hydroxyl group-containing monomer, 2-hydroxyethyl(meth)acrylate and 3-hydroxypropyl (meth)acrylate are preferable, andthese can be used alone or in admixture thereof. In this case, it isdesired that the content of the hydroxyl group-containing monomer in thecomponent (d) is 10 to 80% by weight, preferably 15 to 50% by weight,from the viewpoint of jetting stability in an inkjet printer.

It is desired that the content of the component (a) in the vinyl polymeris 1 to 50% by weight, preferably 5 to 40% by weight, from the viewpointof dispersion stability of the dispersion obtained.

It is desired that the content of the component (b) in the vinyl polymeris 5 to 50% by weight, preferably 10 to 40% by weight, from theviewpoints of jetting stability and dispersion stability.

Also, it is preferable that the total content of the component (a) andthe component (b) in the vinyl polymer is 6 to 75% by weight from theviewpoints of dispersion stability in water and jetting stability.

It is desired that the content of the component (c) in the vinyl polymeris 1 to 25% by weight, preferably 5 to 20% by weight, from the viewpointof increasing the content of the oil-soluble black dye or carbon blackand from the viewpoint of suppressing scorching on the heater surface ofan inkjet printer.

It is desired that the content of the component (d) in the vinyl polymeris 5 to 93% by weight, preferably 10 to 80% by weight, from theviewpoint of suppression of scorching on the heater surface of an inkjetprinter and from the viewpoint of stability. When the component (d)comprising the styrenic monomer is used, it is preferable that thecontent of the component (d) in the vinyl polymer is 10 to 60% byweight. Also, when the component (d) comprising the hydroxylgroup-containing monomer is used, it is preferable that the content ofthe component (d) in the vinyl polymer is 1 to 30% by weight.

The vinyl polymer is prepared by polymerizing the components (a) to (d)by means of a known polymerization method such as a bulk polymerizationmethod, a solution polymerization method, a suspension polymerizationmethod, an emulsion polymerization method, or the like. Among thesepolymerization methods, the solution polymerization method ispreferable.

The solvent used in the solution polymerization method is not limited tospecified ones, and any of water-miscible organic solvents andwater-insoluble organic solvents can be used. Examples of the solventinclude an aliphatic alcohol having 1 to 3 carbon atoms; ketones such asacetone and methyl ethyl ketone; esters such as ethyl acetate; toluene,and the like. Among them, methanol, ethanol, acetone, methyl ethylketone or mixed solvents thereof with water are preferable.

Incidentally, a radical polymerization initiator can be used in thepolymerization. As the radical polymerization initiator, any of azocompounds and organic peroxides may be used. It is preferable that theamount of the radical polymerization initiator is 0.001 to 5% by mol,especially 0.01 to 2% by mol based on the total amount of the components(a) to (d).

In the polymerization, a polymerization chain transfer agent may befurther added. The polymerization chain transfer agent includesmercaptans, xanthogenndisulfides, thiuram disulfides, halogenatedhydrocarbons, and the like. It is preferable that the amount of thepolymerization chain transfer agent is usually 0.001 to 5 parts byweight based on 100 parts by weight of the monomers charged.

The conditions for polymerizing the components (a) to (d) differdepending upon kinds of the radical polymerization initiator, themonomers and the solvent used, and the like. The polymerizationtemperature is usually 30° to 100° C., preferably 50° to 80° C., and thepolymerization time period is usually 1 to 20 hours. In addition, it ispreferable that the polymerization atmosphere is an atmosphere of aninert gas such as nitrogen gas.

After the termination of the polymerization reaction, a copolymer can beisolated from the reaction solution by a known method such asreprecipitation or distilling off of the solvent. Also, the resultingcopolymer can be purified by the removal of unreacted monomers and thelike by the repeat of reprecipitation, membrane separation,chromatography or extraction.

The weight-average molecular weight of the vinyl polymer (as determinedby the method described in Preparation Example 1) is preferably 3000 to200000, more preferably 5000 to 100000, from the viewpoints ofjettability, prevention of scorching on a printer head, durability ofthe water-based ink after printing, and stability of the dispersion.

The oil-soluble black dyes usable in the present invention include C. I.Solvent Black 3, C. I. Solvent Black 7, C. I. Solvent Black 27, C. I.Solvent Black 29, C. I. Solvent Black 34, Oil Black 860, and the like.Also, the carbon black includes Monarch 880, Monarch 280, Monarch 400,Regal 400R and Mogul L (hereinabove manufactured by Cabot Corporation,trade names); PRINTEX-L, PRINTEX-150T, PRINTEX-90, PRINTEX-60 andPRINTEX-80 (hereinabove manufactured by Degussa-AG, trade names); #950,#960 and MA600 (hereinabove manufactured by Mitsubishi ChemicalCorporation, trade names), and the like.

It is desired that the total amount of the oil-soluble black dye and thecarbon black is 20 to 400 parts by weight, preferably 30 to 300 parts byweight, based on 100 parts by weight of the resin solid content of thevinyl polymer, from the viewpoints of optical density and facilitationof inclusion of the black dye or the carbon black into the vinylpolymer.

The aqueous dispersion of the vinyl polymer particles comprising theoil-soluble black dye or the carbon black can be prepared by, forexample,

-   (i) a process comprising dissolving the vinyl polymer together with    the oil-soluble black dye in a hydrophilic organic solvent, adding a    neutralizing agent as occasion demands to cationize or anionize the    salt-forming group of the polymer, adding water to the resulting    mixture, and thereafter distilling off the hydrophilic organic    solvent to phase-invert to a water-based system;-   (ii) a process comprising dissolving the vinyl polymer in an organic    solvent, adding a neutralizing agent as occasion demands to    cationize or anionize the salt-forming group of the polymer, adding    water and carbon black thereto and kneading the mixture, and    distilling off the organic solvent from the kneaded product to give    a water-based system;-   (iii) a process comprising adding water, a neutralizing agent and,    as occasion demands, a surfactant to a solution obtained by    dissolving the vinyl polymer and the oil-soluble black dye in a    water-insoluble organic solvent, to cationize or anionize the    salt-forming group of the polymer, emulsifying the resulting    mixture, and thereafter distilling off the water-insoluble organic    solvent to give a aqueous dispersion; or the like.

The neutralizing agent usable for neutralizing the cationic groupincludes inorganic acids such as hydrochloric acid, nitrous acid,sulfuric acid and phosphoric acid; and organic acids such as acarboxylic acid having 1 to 5 carbon atoms, a dialkyl phosphoric acidhaving 2 to 20 carbon atoms, and oxyacids such as lactic acid, glycolicacid, glyceric acid and gluconic acid. Also, the neutralizing agentusable for neutralizing the anionic group includes alkali metalhydroxides such as sodium hydroxide, potassium hydroxide and lithiumhydroxide; basic substances such as ammonia, triethylamine andmorpholine; alcoholamines such as triethanolamine, diethanolamine andN-methyldiethanolamine, and the like. It is desired that theneutralization degree is adjusted so that the resulting aqueousdispersion becomes neutral.

The average particle diameter of the vinyl polymer particles includingthe oil-soluble black dye or the carbon black is preferably 0.01 to 0.50μm, more preferably 0.02 to 0.20 μm, from the viewpoints of preventionof generation of blotting of the ink and stability of the dispersion.

The amount of the oil-soluble black dye or the carbon black ispreferably 20 to 400 parts by weight, more preferably 30 to 300 parts byweight, based on 100 parts by weight of the solid content of the vinylpolymer, from the viewpoints of optical density and facilitation ofinclusion of the oil-soluble black dye or the carbon black in the vinylpolymer.

It is desired that the amount of the water-insoluble vinyl polymerparticles containing the oil-soluble black dye or the carbon black inthe water-based ink comprising the aqueous dispersion is adjusted sothat the content of the vinyl polymer particles in the water-based inkis usually 1 to 35% by weight, preferably 2 to 15% by weight, from theviewpoints of optical density and jetting stability.

The color ink refers to a color ink other than the black ink, forinstance, an ink comprising a pigment or dye which is yellow, magenta,cyan or the like. The color ink includes, an aqueous pigment dispersioncontaining a dispersant, an aqueous dispersion of polymer particlescontaining a pigment or dye, an aqueous solution or aqueous dispersionof a dye, and the like. In addition, the vinyl polymer particles used inthe black ink has an ionicity opposite to that of the color ink.Specifically, for instance, when the ionicity of the vinyl polymerparticles used in the black ink is cationic, the ionicity of the colorink is anionic, and when the ionicity of the vinyl polymer particles ofthe black ink is anionic, the ionicity of the color ink is cationic.

Here, the ionicity is determined, for example, by potentiometrictitration or determination of pH in the case of a water-soluble dye, orby determination of zeta potential in the case of a dispersion.

Since the ink is excellent in bleeding resistance (no blotting isgenerated on its periphery) based on that the ionicity of theoil-soluble black dye or the carbon black is opposite to that of thecolor ink, very clear printouts can be obtained.

The ink set of the present invention may be either a combination inwhich an oil-soluble black dye or carbon black used in the black ink isanionic, and the color ink is cationic; or a combination in which anoil-soluble black dye or carbon black used in the black ink is cationic,and the color ink is anionic. Many of anionic dyes show good colordevelopment, and especially acidic dyes are preferable. Therefore, it ispreferable that the black ink is cationic when the color ink is anionic,from the viewpoint of broadening the selection of dyes.

Further, it is preferable to use an aqueous dispersion of vinyl polymerparticles containing a dye or pigment as the color ink from theviewpoints of water resistance and high fixing ability.

Various additives which have been generally widely used, for instance, awetting agent such as a polyhydric alcohol, a dispersant, a defoamingagent, a mildewproof agent, a chelating agent, and the like can beproperly added to the black ink and the color ink in the presentinvention.

Since the aqueous dispersion of the vinyl polymer particles containingthe oil-soluble black dye or the carbon black as described above isexcellent in fixing ability, the aqueous dispersion is excellent inrubbing resistance, high-lighter fastness, and water resistance, andshows little blotting.

EXAMPLES

The units “parts” and “%” as used in each of Examples and ComparativeExamples are based upon weight.

Preparation Example 1

A reaction vessel was charged with 20 parts of methyl ethyl ketone as apolymerization solvent, monomers and a polymerization chain transferagent which are listed in the column of initially charged monomers ofTable 1, and nitrogen gas replacement was sufficiently carried out.

On the other hand, after a dropping funnel was subjected to sufficientnitrogen gas replacement, the dropping funnel was charged with monomersand a polymerization chain transfer agent which are listed in the columnof dropping monomers of Table 1, 60 parts of acetone and 0.2 parts of2,2′-azobis(2,4-dimethylvaleronitrile). The temperature of the mixedsolution inside the reaction vessel was raised to 65° C. with stirringunder nitrogen atmosphere, and the mixed solution inside the droppingfunnel was gradually added dropwise thereto over a period of 3 hours.After 2 hours passed from the termination of dropping, thereto was addeda solution prepared by dissolving 0.1 parts of2,2′-azobis(2,4-dimethylvaleronitrile) in 5 parts of methyl ethylketone. The mixture was further aged at 65° C. for 2 hours and then at70° C. for 2 hours, to give a copolymer solution.

A part of the resulting copolymer solution was isolated by drying thesolution at 105° C. for 2 hours under reduced pressure to completelyremove the solvent. The molecular weight was determined by gelpermeation chromatography using polystyrene as a standard substance, andchloroform containing dodecyldimethylamine of 1 mmol/L as a solvent. Asa result, the copolymer had a weight-average molecular weight of 10000.

The resulting copolymer solution was dried under reduced pressure. To 5parts of the resulting copolymer were added 25 parts of an organicsolvent listed in Table 1 and 5 parts of an oil-soluble black dye listedin Table 1, and completely dissolved. To the resulting solution wasadded a neutralizing agent (30% aqueous solution) listed in Table 1 withstirring to partly neutralize a salt-forming group of the copolymer.Thereto was added 300 parts of ion-exchanged water, and thereafter themixture was emulsified using Microfluidizer (manufactured byMicrofluidizer) for 30 minutes.

The organic solvent was completely removed from the resulting emulsionat 60° C. under reduced pressure. Further, the mixture was concentratedby removing a part of water, to give a cationic aqueous dispersion ofvinyl polymer particles containing an oil-soluble black dye (hereinafterreferred to as “BK-1”), the concentration of the solid ingredients ofwhich was 10%.

Preparation Example 2

A copolymer solution was obtained in the same manner as in PreparationExample 1 using the monomers and a polymerization chain transfer agentwhich are listed in Table 1. The molecular weight was determined in thesame manner as in Preparation Example 1. As a result, the copolymer hada weight-average molecular weight of 10000.

Five parts of a copolymer obtained by drying the resulting copolymersolution under reduced pressure was dissolved in 25 parts of an organicsolvent listed in Table 1. To the resulting solution was added aneutralizing agent (30% aqueous solution) listed in Table 1 withstirring to partly neutralize a salt-forming group of the copolymer.Thereto were further added 50 parts of ion-exchanged water and 5 partsof carbon black listed in Table 1, and the mixture was kneaded with abeads-mill. The organic solvent was completely removed from theresulting kneaded mixture at 60° C. under reduced pressure. Further, themixture was concentrated by removing a part of water, to give a cationicaqueous dispersion of vinyl polymer particles containing carbon black(hereinafter referred to as “BK-2”), the concentration of the solidingredients of which was 10%.

Each compound listed in Table 1 means the followings:

-   1) silicone macromer: manufactured by CHISSO CORPORATION under the    trade name of FM-0711;-   2) styrenic macromer: manufactured by TOAGOSEI CO., LTD. under the    trade name of AS-6 [styrene homopolymerized macromer having a    number-average molecular weight of 6000]; and-   3) styrene-acrylonitrile macromer: manufactured by TOAGOSEI CO.,    LTD. under the trade name of AN-6 [styrene content: 75% by weight,    number-average molecular weight: 6000].

TABLE 1 Prep. Ex. No. 1 (BK-1) 2 (BK-2) Initially Charged t-ButylMethacrylate (20) Styrene (9) Monomers and N,N-Dimethylaminoethyl LaurylMethacrylate (2) Polymerization Methacrylate (20) N,N-DimethylaminoethylChain Transfer Methoxypolyethylene Methacrylate (15) Agent Glycol (n =9) Methoxypolyethylene (parts) Methacrylate (5) Glycol (n = 4)2-Hydroxyethyl Methacrylate (15) Methacrylate (2) Styrenic MacromerSilicone Macromer AS-6 (5) FM-0711 (2) Mercaptoethanol (0.2)n-Dodecylmercaptan (0.2) Dropping t-Butyl Methacrylate (20) Styrene (10)Monomers and N,N-Dimethylaminoethyl Lauryl Methacrylate (2)Polymerization Methacrylate (20) N,N-Dimethylaminoethyl Chain TransferMethoxypolyethylene Methacrylate (15) Agent (parts) Glycol (n = 9)Methoxypolyethylene Methacrylate (5) Glycol (n = 4) 2-HydroxyethylMethacrylate (20) Methacrylate (2) Styrenic Macromer Silicone MacromerAS-6 (5) FM-0711 (2) Mercaptoethanol (1.8) n-Dodecylmercaptan (1.8)Dye/Pigment Oil Black 860*¹ Carbon Black Monarch 880^(*2) OrganicSolvent Acetone Methyl Ethyl Ketone Neutralizing Glycolic Acid (3.2)Gluconic Acid (5.2) Agent (Note) *1: manufactured by Orient ChemicalCo., Ltd., trade name *2: manufactured by Cabot Corporation, trade name

Preparation Example 3

Ten parts of carbon black having a surface area of 230 m²/g and a DBPoil absorption of 70 m²/g and 3.06 parts of 3-amino-N-ethylpyridiniumbromide were thoroughly mixed with 72 parts of water. Thereafter, 1.62parts of nitric acid was added dropwise thereto, and the mixture wasstirred at 70° C. After 4 minutes passed, a solution prepared bydissolving 1.07 parts of sodium nitrite in 5 parts of water was addedthereto, and the mixture was stirred for additional 1 hour. Theresulting slurry was filtered with Toyo Roshi No. 2 (manufactured byADVANTICE), and the carbon black particles were sufficiently washed withwater. This wet cake-like carbon black was re-dispersed in 3000 mL ofwater, and desalted with a reverse osmotic membrane until the electricconductivity attained to 0.2 μs. Further, this liquid dispersion ofcarbon black was concentrated to a carbon black concentration of 10%,thereby giving a cationic, self-dispersible carbon black aqueousdispersion, to the surface of which a group represented by the formula:

was bonded (hereinafter referred to as BK-3).

Example 1

A black ink comprising a cationic colorant composed of the followingcomponents and anionic color inks of yellow, magenta and cyan composedof the following components were combined together to give an ink set.Each ink was prepared by mixing each of the components, and filteringwith a microfilter having a pore size of 5 μm as to the black ink orwith a microfilter having a pore size of 0.45 μm as to the other inksunder pressure, to give an ink.

[Components] 1) Black Ink BK-1 80 parts Glycerol 5 parts Ethylene Glycol5 parts Isopropyl Alcohol 2 parts Acetylenol EH (manufactured by Kawaken2 parts Fine Chemicals Co., Ltd., trade name) Water 6 parts 2) ColorInks <Yellow> C.I. Acid Yellow 25 4 parts Glycerol 5 parts DiethyleneGlycol 5 parts Urea 5 parts Sodium polyoxyethylene lauryl ether sulfate1 part [manufactured by Kao Corporation under the trade name of EMAL20C] Water 80 parts <Magenta> C.I. Acid Red 51 4 parts Glycerol 5 partsε-Caprolactam 5 parts Urea 5 parts EMAL 20C [manufactured by KaoCorporation, 1 part trade name] Water 80 parts <Cyan> C.I. Acid Blue 594 parts Glycerol 5 parts 2-Pyrrolidone 5 parts Urea 5 parts EMAL 20C[manufactured by Kao Corporation, 1 part trade name] Water 80 parts

Example 2

The same procedures as in Example 1 were carried out except that theblack colorant BK-1 was changed to BK-2 in the black ink, to give an inkset.

Comparative Example 1

The same procedures as in Example 1 were carried out except that ananionic black ink composed of the following components was used as theblack ink, to give an ink set.

1) Black Ink C.I. Direct Black 195 4 parts Glycerol 5 parts2-Pyrrolidone 5 parts Urea 5 parts Isopropyl Alcohol 2 parts EMAL 20C[manufactured by Kao Corporation, 1 part trade name] Water 78 parts

Comparative Example 2

The same procedures as in Example 1 were carried out except that acationic black ink composed of the following components was used as theblack ink, to give an ink set.

1) Black Ink BK-3 4 parts Glycerol 5 parts Urea 10 parts IsopropylAlcohol 2 parts Acetylenol EH (manufactured by Kawaken 1 part FineChemicals Co., Ltd., trade name) Water 78 parts

The physical properties for the ink sets obtained in Examples 1 and 2and Comparative Examples 1 and 2 were evaluated by the followingmethods. The results are shown in Table 2.

<Evaluation Methods>

(1) Printing Property of Black Ink

An extent of distortion was observed with naked eyes when ruled lineswere printed on a regenerated paper for PPC (manufactured by Nippon KakoSeishi K.K.) using a commercially available bubble jet printermanufactured by CANON INC. (model number: BJC-430), and evaluated on thebasis of the following evaluation criteria.

(Evaluation Criteria)

-   ⊚: No distortion-   ◯: Little distortion-   Δ: Some distortion-   x: Distortion    (2) Water Resistance of Black Ink

Solid image printing was carried out with a black ink on the regeneratedpaper for PPC by using the above-mentioned printer, and dried at roomtemperature (about 25° C.) for 1 hour. Thereafter, the printed copypaper was immersed in stand-still water for 10 seconds, and the paperwas vertically lifted therefrom. After drying the paper in the air atroom temperature (about 25° C.), the residual ratio of the opticaldensity after immersion to the initial optical density was obtained. Thewater resistance was evaluated on the basis of the following evaluationcriteria:

(Evaluation Criteria)

-   ⊚: Residual ratio being at least 95%-   ◯: Residual ratio being at least 90% and less than 95%-   Δ: Residual ratio being at least 70% and less than 90%-   x: Residual ratio being less than 70%    (3) Rubbing Resistance of Black Ink

Solid image printing was carried out with a black ink on the regeneratedpaper for PPC using the above-mentioned printer. After the paper wasdried for 1 day at room temperature (about 25° C.), the printed surfacewas strongly rubbed with a finger. The extent of rub-off of the printedimage was observed with naked eyes, and evaluated on the basis of thefollowing evaluation criteria:

(Evaluation Criteria)

-   ◯: Substantially no printed image being rubbed off, and its    periphery being not stained.-   Δ: Some printed images being rubbed off, its periphery being    stained, and finger also being stained a little.-   x: Printed images being considerably rubbed off, its periphery being    considerably stained, and finger also being considerably stained.    (4) High Lighter-Fastness of Black Ink

Text printing was carried out with a black ink on the regenerated paperfor PPC by using the above-mentioned printer. After 6 hours passed, theextent of staining of the printed sample was observed with naked eyesafter being traced with a commercially available aqueous fluorescentmarker, and evaluated on the basis of the following evaluation criteria:

(Evaluation Criteria)

-   ◯: No stains such as rubbed stains were observed even when traced    with a fluorescent marker.-   Δ: Some rubbed stains were generated when traced with a fluorescent    marker, but such stains would cause no problems in practical uses.-   x: Generation of rubbed stains was observed when traced with a    fluorescent marker, and the stains were considerably noticeable.    (5) Bleeding Resistance Between Black Ink and Color Inks

Solid image printing was carried out with a black ink out of each inkset on the regenerated paper for PPC by using the above-mentionedprinter. Immediately thereafter, solid image printing of each color inkwas carried out, so that an adjoining portion to the previous solidimage was printed with a yellow, magenta or cyan ink. The boundaryportion of the resulting solid image printing was observed with nakedeyes, and the bleeding between the black ink and the color ink wasevaluated on the basis of the following evaluation criteria.

(Evaluation Criteria)

-   ⊚: No bleeding being recognized on its entire periphery-   ◯: Slight bleeding being recognized but not to be noted-   Δ: Some bleeding being recognized-   x: Noticeable bleeding being recognized on almost the entire    periphery

TABLE 2 High Printing Water Rubbing Lighter- Property of ResistanceResistance Fastness Black Ink of of of Bleeding (Distortion) Black InkBlack Ink Black Ink Resistance Ex. No. 1 ⊚ ⊚ ∘ ∘ ∘ 2 ∘ ⊚ ∘ ∘ ∘ Comp. Ex.1 ⊚ x ∘ Δ x 2 Δ Δ x x ∘

It can be seen from the above results that all of the ink sets obtainedin Examples are excellent in printing property, water resistance,rubbing resistance and high-lighter fastness of the black ink, and alsoare excellent bleeding resistance.

INDUSTRIAL APPLICABILITY

Since the ink set of the present invention is excellent in jettability,water resistance and fixing ability, and further excellent in bleedingresistance and can form very clear printed characters, the ink set canbe suitably used as a water-based ink for inkjet recording.

1. An ink set comprising a black ink and a color ink, wherein the black ink consists essentially of an aqueous dispersion of anionic water-insoluble vinyl polymer particles containing (i) a vinyl polymer and (ii) carbon black, and wherein the water-insoluble anionic vinyl polymer particles have an ionicity opposite to that of the color ink, and said vinyl polymer comprises a water-insoluble vinyl polymer prepared by copolymerizing (a) a salt forming group-containing monomer, (b) a monomer represented by the formula (I): CH₂═C(R¹)COO(R²O)_(p)R³   (I) wherein R¹ is hydrogen atom or a lower alkyl group, R² is a divalent hydrocarbon group which may have a hetero-atom, R³ is a monovalent hydrocarbon group which may have a hetero-atom, and p is a number of 1 to 60, (c) a styrenic macromer having a polymerizable functional group at one end, and (d) a styrenic monomer copolymerizable with these monomers.
 2. The ink set according to claim 1, wherein the styrenic macromer is a styrene homopolymer having a polymerizable functional group at one end.
 3. The ink set according to claim 1, wherein the styrenic macromer has a number-average molecular weight of 1000 to
 10000. 4. The ink set according to claim 1, wherein the styrene monomer is at least one styrenic monomer selected from the group consisting of styrene, vinyltoluene, 2-methylstyrene and chlorostyrene. 